Recently, semiconductor devices having higher speed of operation and a higher degree of integration have been developed, and the degree of integration in LSIs has been significantly improved. Consequently, heat radiation of the substrate material on which semiconductor elements are mounted has come to be of greater importance.
Alumina (Al.sub.2 O.sub.3) has been used as a ceramics for IC boards. However, since the conventional alumina sintered body has a low thermal conductivity, the heat radiation is not sufficient. Therefore, it has become more difficult to use alumina sintered bodies as ceramics for substrates which can sufficiently cope with the increase of heat emitted from IC chips.
In view of the foregoing, substrates, heat sinks and packages using aluminum nitride having high a thermal conductivity have come to attract attention as a good substitute for alumina substrates. Research has been made for practical applications of aluminum nitride.
Aluminum nitride is a material which inherently has a high thermal conductivity and a high electrical insulation but unlike beryllia porcelain which also has a high thermal conductivity, aluminum nitride has no toxicity. Therefore, it is considered as a promising insulating material or packaging material for semiconductor devices.
In a conventional method of processing circuit boards formed of alumina, a large thin plate is formed by a sheet forming method, the sheet is provided with grooves forming rated break lines by laser processing, and the thin plate is divided into pieces by fracturing along said rated break lines. Substrates formed of alumina have been processed variously by using laser, and the substrates have been widely used as insulating substrates for ICs.
Similar laser processing has been proposed for processing substrates formed of aluminum nitride. However, aluminum nitride is decomposed into Al and N.sub.2 gas by laser irradiation. Consequently, Al is deposited at portions processed by the laser, degrading the insulation of the aluminum nitride substrate. A conductive layer as an electric resistant body is formed on a circuit board formed of aluminum nitride. In order to adjust the electric resistance value of the electric resistant body, the conductive layer is selectively removed by laser irradiation. In this case also, Al is deposited at portions processed by the laser, causing fluctuation of the electrical resistantce value of the electric resistant body. Therefore it becomes difficult to adjust the electric resistance value of the electric resistant body to a desired value.
In order to solve such problems, a method is disclosed in Japanese Patent Laying-Open No. 63-131501 in which oxygen is supplied to portions processed by a laser during laser processing or after laser processing, so as to oxidize any deposited Al and to form alumina. This method is said to enable the adjustment of the resistance value of the electric resistant body selectively removed by laser irradiation to a prescribed value.
As disclosed in Kino Zairvo (functional material) October, 1987, p. 60, laser processing and the selection of laser conditions in water have been proposed to prevent a decrease of the insulation resistance at portions irradiated by a laser and resulting from a decomposition of aluminum nitride by laser energy.
However, so long as the aluminum sintered body is processed by laser irradiation, deposition of Al at portions irradiated by the laser cannot be perfectly avoided even if the best processing conditions are selected. Consequently, it is often necessary to remove the deposited Al after processing by using an acid or an alkali solution.
Especially, irradiation by a laser of an aluminum nitride sintered body itself causes corrosion of the aluminum nitride sintered body, and circuit portions formed on the surface of the aluminum nitride sintered body are affected. Laser processing of circuit boards formed of aluminum nitride sintered body has various difficulties. The scribing of, trimming of, or forming holes in aluminum nitride substrates are difficult in practice so long as laser irradiation is employed.